Optical fibers are often used for communication purposes, where light waves can propagate in the fiber over long distances with low or no loss. However, by enhancing the sensitivity of the light properties to environmental influences, the optical fibers can be used to detect or monitor external perturbations, such as temperature or stress.
Such optical fiber sensors can be implemented as point sensors, where only one location along the optical fiber is made sensitive to the external perturbations. Accordingly, one optical fiber is needed per point, which is to be monitored. Alternatively, the fiber optical sensors can be implemented as distributed sensors, where the optical fiber is a long uninterrupted linear sensor.
When the power of the propagated light exceeds a given threshold, non-linear phenomena, such as Brillouin scattering starts to occur. Due to its strong dependence on the aforementioned environmental variables, Brillouin scattering is often employed in distributed optical fiber sensor systems.
Brillouin scattering occurs due to the interaction between an electromagnetic wave and matter, which can generate variations in the molecular structure of the material. The incident light wave generates acoustic waves and induces a periodic modulation of the refractive index, which in turn forms a light-backscattering similar to a Bragg grating. The scattered light is down-shifted in frequency due to the Doppler shift associated with the grating moving at the acoustic velocity. The acoustic velocity is dependent on the density of the material. The density of the material is temperature-dependent as a result of thermal expansion so that a peak frequency of the interaction is observed to change with temperature. Further, any deformation experienced by the fiber will also have an impact on the density of the material, whereby the fiber can be used as a distributed strain gauge by observing a shift when the fiber is elongated.
By using different time domain or frequency correlation techniques, the Brillouin shift process can accurately be located along the optical fiber.
C. A. Galindez-Jemioy and J. M. López-Higuera, “Brillouin Distributed Fiber Sensors: An Overview and Applications”, Journal of Sensors, Volume 2012 is a review article that provides an overview and applications of various Brillouin sensor setups, which are incorporated in the present invention by reference.
Luc Thévenaz, “Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives”, Front. Optoelectron. China, Higher Education Press and Springer Verlag Berlin Heidelberg, 2010 is another review article that provides an overview and applications of various Brillouin sensor setups, which are incorporated in the present invention by reference.
The Brillouin based sensor systems today utilize standard single-mode fibers. However, such fibers are attributed with a poor Brillouin gain coefficient and unwanted nonlinear effects that may cause modulation instability. Accordingly, there is a need for improved distributed Brillouin sensor systems.